As a method of measuring the concentration of a substance included in a sample solution, a method that uses an optical waveguide effect is known. According to an exemplary method, a laser beam enters an optical waveguide layer through an entrance-side grating to thereby generate evanescent waves, and light emitted from an exit-side grating is detected. The power of the evanescent waves changes due to a reaction between substances included in a sample solution and molecules included in a functional film formed on the optical waveguide layer. The concentration of the substance included in the sample solution is measured based on the change in the power of the evanescent waves, which is obtained by detecting the light emitted from the exit-side grating.
A device that measures the concentration of a substance included in a sample solution using such an optical waveguide effect is called an optical waveguide sensor. The optical waveguide sensor includes a transparent substrate formed of glass or quartz, for example, a pair of gratings formed within the substrate on a principal surface of the substrate so as to allow a laser beam to enter and exit, an optical waveguide layer formed of polymer resin having a higher refractive index than the substrate on a principal surface of the substrate including the gratings, and a functional film formed on the optical waveguide layer. In such an optical waveguide sensor, a laser beam enters an optical waveguide from the substrate side through the substrate. Therefore, the substrate needs to be a transparent substrate that transmits the laser beam, and the material to be used as the substrate is limited. Moreover, since the laser beam enters the optical waveguide through the substrate, it is necessary to perform optical design taking the refractive indices of a plurality of materials into account. In addition, the laser beam loses its power when it passes through the substrate.
In the optical waveguide sensor of the related art, since the laser beam enters the optical waveguide through a transparent substrate, there is a problem in that it is necessary to perform optical design taking the refractive indices of a plurality of materials into account, which complicates the optical design. Another problem is that the laser beam loses its power when it passes through the substrate.